Polyimide or polybenzoxazole is excellent in thermal resistance, mechanical properties, chemical resistance, electrical insulation properties and the like, and thus, is expected of extended usage in the electrical and electronic applications such as semiconductor surface protecting film, interlayer insulating film in multilayer wiring board, cover coat film for flexible wiring board, and the like. In particular, photosensitive polyimide or photosensitive polybenzoxazole imparted with a photoresist function is a material attracting great interest, in view of shortening the fabrication process significantly.
In recent years, high speed and high capacity of computers or measurement hardware are required even further in the field of communication and information processing technology. Thus, increase in the signal transmission speed in printed circuit boards used therein is still in progress, and a demand for the lowering of dielectric constant is also increasing. Also, due to the concern about the safety problems during operation or the environmental impact, a positive photosensitive resin composition which is capable of development using an aqueous alkali solution is strongly desired.
Heretofore, suggestions have been made for the method of exhibiting positive photosensitivity, which include a method of introducing an o-nitrobenzyl group to a polyimide precursor via an ester bonding (see, for example, Non-Patent Document 1), a method of adding a dissolution inhibitor containing naphthoquinone diazide to a polybenzoxazole precursor or polyhydroxyimide (see, for example, Patent Document 1 and Non-Patent Document 1), and the like. However, the high solubility of the polyimide precursors in alkali water causes problems such as that sufficient resolution cannot be achieved, the structure is restricted to particular structures only, and film shrinkage of the finally obtained film causes a problem. Thus, none of the methods has been put to practical use.
So far, there have been suggested a variety of photosensitive polyimide skeletons. However, the wholly aromatic polyimide precursor reported in Patent Document 2 has problems such as its solubility in alkali developing solutions, or insufficient transparency in the i-line region. Furthermore, a polyimide precursor having an acid dianhydride in the alicyclic form has problems such as insufficient reactivity of the acid dianhydride, susceptibility to coloration, and high price due to the special structure of the acid dianhydride of the monomer. A wholly alicyclic polyimide precursor has a problem of unsatisfactory film formation, while a chain-like aliphatic polyimide has a problem of poor thermal resistance (See, for example, Non-Patent Document 2).
Therefore, a polyimide precursor which has adequate solubility in alkalis, thermal resistance, and high transparency in the i-line region, and which uses an inexpensive monomer, is desired.
Moreover, in recent years, there is an increasing demand for flexible wiring boards in association with diversification of electronic instruments, such as miniaturization, sophistication and the like. A flexible wiring board is generally manufactured by producing a circuit on a flexible printed wiring board in which an electrically insulating film and a metal foil are integrated by lamination, using an adhesive if necessary, and a coverlay is coated over this flexible wiring board for the purpose of protection of the circuit.
The coverlay is required to have thermal resistance, folding resistance and electrical insulation properties, and thus polyimide films have been used therefor. A means to adhere the coverlay that is used in general is a method of laminating a coverlay with desired perforations on a flexible wiring board on which a circuit pattern is formed, by thermal lamination or by pressing.
However, recently, the materials for mounting are required to have even higher performance than conventional materials, and the flexible printed wiring boards are also in rapid progress for micronization of the wiring. The method of aligning in this manner is facing limits in view of low yield, poor workability, difficulty in obtaining accuracy of position or having low accuracy, and the like.
Therefore, in order to solve such problems, a photosensitive insulating resin composition which is applicable to the method of photolithography enabling microfabrication, is being investigated for the use as a photosensitive coverlay. These photosensitive coverlays are provided as dry film type and liquid type, and both types are currently in practical use.
The dry film type photosensitive coverlay is mainly used in a form that a solution of a resin composition containing a photosensitive polyimide or a polyimide precursor in an organic solvent, is coated on a support film and dried, and a protective film such as polyethylene or the like is laminated thereon to prevent any dust attachment. While this type of coverlay has a merit of easy handlability, it is general to use a polymer having an acrylic or methacrylic skeleton with carboxyl group for the coverlay in order to allow development in an aqueous alkali solution, and the polymer has a disadvantage of low thermal resistance or flex resistance after curing (See, for example, Patent Document 3). Furthermore, in recent years, thinner flexible printed wiring boards are desired, and there is a problem in the dry film method that use of support films makes it difficult to reduce the film thickness down to 50 μm or less.
For the liquid type photosensitive coverlay, photosensitive polyimide precursors which result in polyimides imparted with photosensitivity have been suggested previously; however, there is a problem in the handling in an industrial scale because an organic solvent such as dimethylsulfoxide, N-methylpyrrolidone or the like is required in the development process.
Thus, solder resists for printed circuit board have been dominantly used as a photosensitive material that is capable of development in a dilute aqueous alkali solution, but it is difficult to say that the material, which is epoxy-acrylic-based, has sufficient flexibility or thermal resistance (See, for example, Patent Document 4). Also, a resin material used for the coverlay for flexible printed wiring board is required to have sufficient flame retardance property, in addition to thermal resistance, folding resistance and insulation properties. However, according to the reports to date, the current situation is such that halogen-containing compounds such as bromine-containing aromatic compounds or the like, which has high possibility of generating dioxin upon combustion, are used, or toxic antimony compounds are used, for the purpose of exhibiting flame retardance.
As examined above, it can be said that it is preferable to use polyamic acid which can be developed by dissolving in aqueous alkali solutions before thermal curing, and which has alkali-soluble carboxylic acid that disappears by ring-closure after thermal curing. However, conventional polyamic acids have difficulties in controlling the dissolution rate in alkali developing solutions, and in exhibiting a difference in the dissolution rate between the exposed part and the unexposed part. Furthermore, although a film thickness of 10 μm or greater is required to exhibit excellent insulation reliability, there is a problem of very low transmittance for actinic ultraviolet ray with this thickness, when conventional polyamic acids are used.
[Patent Document 1] JP-A 11451/1993
[Patent Document 2] JP-A 161110/1994
[Patent Document 3] JP-A 167336/2003
[Patent Document 4] JP-A 232195/2005
[Non-Patent Document 1] J. Macromol. Sci. Chem., A24, 10, 1407, 1987
[Non-Patent Document 2] Recent Polyimides, NTS Corp., 2002